How do you count the number of set bits in an integer?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
Brian Kernighan's trick repeatedly does n &= n - 1, clearing the lowest set bit until n is zero
WHY — Bit Manipulation instead of guessing?
Why interviewers care about Bit Manipulation:
question about Bit Manipulation.
trade-offs, and what you would actually do on a DSA project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Brian Kernighan's trick repeatedly
does n &= n - 1, clearing the lowest set bit until n is zero
- 2Why it exists
that is O(number of 1-bits).
- 3Shifting and masking every
bit is O(word size), typically 32 or 64.
- 4Give an example
Both use O(1) extra space.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Brian Kernighan's trick repeatedly does n &= n - 1, clearing the lowest set bit until n is zero that is O(number of 1-bits).